Attic Ventilation Net Free Area

Calculate or enter the square footage of the attic or area to be vented.

Attic ventilation net free area.

The total net free area of the intake and exhaust must be balanced. Of net free area of intake needed at or near the soffit. Besides the rated air ventilation area described by various vent product manufacturers other roof and vent opening details can significantly affect the actual airflow and level of under roof ventilation at a building. If you do not have upper vents within 3 ft of the ridge then use 1 150.

Nfa is the unobstructed area through which air can pass through a vent usually measured in square inches. Should you need to add more air intake ventilation to your attic simply determine the number of vents needed from the table based on your net free area requirements. The net free area is always less than the actual area. This calculates the 2015 international residential building code minimum irc section r806 roof ventilation 1 which states that 1 square foot of net free area for every 150 square feet of attic floor space with the attic defined as length x width floor of the attic.

The calculations before indicated that we needed 2 square feet of ridge or upper roof ventilation. Eave vent gable vent roof jack soffit vent screen factor sf mesh sf w o louvers sf w louvers 1 4 or 1 2 1 0 2 0 1 8 1 25 2 25 1 16 2 0 2 0 nfa for common attic vents attic vents nfa. Copper as well as a variety of pre. Check with your building official for current code and a copy of the section regarding roof ventilation.

Two square feet equals 288 square inches. Let owens corning roofing help you calculate exactly how much ventilation you will need for a healthy and balanced attic with our 4 step ventilation calculator. Helpful hint for the 1 150 attic ventilation requirement simply double the amount of exhaust and intake calculated. The passive vent net free area table gives the net free area ventilation for some common roof and gable passive vents.

A normal metal pot vent produces only 50 squares inches of free net area. Attic square footage 2 square inches of exhaust and square inches of intake net free area nfa needed. So we would need at least six metal vents in addition to the lower soffit ventilation to meet minimum ventilation requirements. This attic vent provides maximum airflow 72 square inches of net free ventilation area nfva without detracting from the aesthetics of the roof design.

Screen factor sf adjusts for airflow obstructed by screens and louvers.